1. Field of the Invention
The present invention relates generally to hydraulic braking systems of motor vehicles and, more particularly, to fluid pressure regulating devices to be incorporated in such braking systems.
More particularly, this invention relates to load-responsive pressure proportioning reducing valve assemblies which distribute the hydraulic brake pressure, corresponding to a transient weight transfer from rear wheels of the vehicle to the front wheels caused by the braking operation, by varying the starting pressure for brake fluid pressure reduction supplied to the rear brakes in response to load and deceleration conditions of the vehicle.
2. Discussion of the Prior Art
With regard to wheeled vehicles which have brake systems with front wheel brakes and rear wheel brakes, it is well known that when the vehicle is subjected to braking operations, a load shift phenomenon occurs wherein the share of the vehicle load borne by the front wheels under normal unbraked conditions dynamically increases and the share of the load borne by the rear wheels decreases. This is due to inertia forces which are proportionate to the amount of deceleration of the vehicle produced by the braking operation. In consequence of this phenomenon, the braking ability of the front wheels thus increases and the braking ability of the rear wheels decreases.
In order to correct this condition and prevent locking of the rear wheels with brake application, it is desirable to either increase the hydraulic pressure supplied to the front wheel brakes in proportion to the deceleration of the vehicle while the hydraulic fluid pressure generated in the master cylinder is being supplied directly to the rear wheel brakes, or to reduce the hydraulic pressure supplied to the rear wheel brakes in proportion to the deceleration of the vehicle while the hydraulic fluid pressure generated in the master cylinder is supplied directly to the front wheel brakes. The present invention relates to the latter type of pressure proportioning systems.
A number of different techniques and structures have been developed over the years to reduce the hydraulic pressure being supplied to the rear wheel brakes in proportion to the deceleration of the vehicle. Some of these prior art devices provide effective braking power distribution corresponding to various degrees of deceleration of the vehicle, but only under one given condition of vehicle load. It is desirable to obtain effective distribution of front and rear wheel braking power to prevent premature locking of the rear wheels at such time that excessive braking occurs over an entire range of possible different vehicle load conditions to thereby prevent the occurrence of vehicle spinning.
For commercial vehicles such as trucks and the like, which are subjected to large variations in load conditions, it is common to incorporate into the proportioning reducing valve assembly a device which indirectly senses the weight of the vehicle load or the total weight of the loaded vehicle and correspondingly suppresses the hydraulic pressure supplied from the proportioning reducing valve to the vehicle rear brakes. These devices increase the level of hydraulic pressure required to start the pressure reducing operation of the reducing valve in proportion to the vehicle load to provide effective front and rear wheel braking power distribution which corresponds respectively to various decelerations caused by braking operations under any various given weight conditions.
Many of the older devices utilized for indirectly sensing the vehicle load use the technique of automatically detecting the share of load borne by the rear wheels of the vehicle. These systems have been considered undesirable, as their construction is such that they are subject to installation errors which may create large errors in detecting the vehicle load as a very high precision is demanded for attaining the proper installation load of the control spring used in these reducing valves. In addition, large physical differences in the type and model of vehicles manufactured make it very difficult for universal installation of this type of vehicle load detection device, thereby further increasing the change of error.
Also, these devices generally have linking mechanisms which are undesirably exposed to the exterior elements causing eventual functional deterioration. In addition, difficult adjustments must be made to these systems at the time of installation, and these systems tend to be sensitive to the unevenness or roughness of the road surface.
In contrast to the aforementioned systems for detecting vehicle load conditions, a more preferable technique is the deceleration sensing type load-responsive valve. Valves of this type are constructed such that the hydraulic pressure level selected to start the pressure reduction operation of the proportioning valve is accomplished by sensing a specific deceleration of the vehicle. In most of these devices, a specific deceleration value under unloaded conditions is detected. This type of load-responsive reducing valve assembly is generally designated as the fixed deceleration sensing type. It detects a fixed or specific deceleration value irrespective of the vehicle load condition. A heavier vehicle load condition will require a greater applied hydraulic brake pressure to attain the specific deceleration value than would be the case with unloaded vehicle conditions thereby producing front and rear wheel braking power distribution which is adapted to the condition of load on the vehicle.
Other deceleration sensing type load-responsive reducing valves are of the variable deceleration sensing type which sense the specific deceleration of the vehicle as determined by the various conditions of load on the vehicle respectively. The pressure reducing valve assembly of the present invention relates to the variable deceleration detection type. The most novel systems of this type as presently existing in the prior art are constructed such that the specific hydraulic pressure which is detected by the action of the deceleration sensing means is caused to act directly itself on one end of the pressure proportioning valve to accordingly vary the starting pressure of the pressure proportioning reduction operation of the reducing valve.
In this type of valve assembly, a deceleration control means or device is utilized for the purpose of controlling the action of the deceleration sensing device. The deceleration sensing device generally consists of a spherical member which travels up an incline to block the passage of hydraulic fluid, in excess of a detected level, into the control chamber wherein the fluid under pressure biases the proportioning valve to set the reduction starting pressure. A differential piston is utilized as the device to control the deceleration sensing means. This differential piston has two slightly different effective pressure receiving areas which oppose each other and are made of liquid sealing members which are formed of a high molecular elastic material.
This deceleration control means responds to a predetermined level of hydraulic pressure supplied to the valve assembly from the master cylinder and accordingly in turn controls the operation of the deceleration sensing device. This deceleration control means must respond quickly to a change in hydraulic pressure acting on the two effective pressure receiving areas thereof and thereby accurately control the actuation point of the deceleration sensing means. However, this pressure sensing deceleration control means, because of the aforesaid two liquid sealing members, has a disadvantage in that an increase in the sliding resistance is induced as the level of hydraulic pressure acting thereon increases. This, in turn, attenuates the speed of operation of the deceleration control means such that the deceleration sensing means may thus sense a deceleration lower than the actual vehicle deceleration.
The pressure proportioning reducing valve of the present invention relates to a novel variable deceleration sensing type valve which is void of these disadvantages by the provision of a pressure sensing deceleration control means which is made up of only one liquid sealing member of a high molecular elastic material.